1. Field of the Invention
This invention relates to high efficiency lighted displays, including Liquid Crystal Display (LCD) panels and inexpensive lighted signs for advertising. Embodiments that are suitable as status indicators in hostile environments such as automotive and aviation are also taught. The invention also teaches the methods of a novel art medium. This application is related to Disclosure Document 376495 filed in the Patent and Trademark Office on May 18, 1995. The contents of Disclosure Document 376495 are hereby incorporated by reference herein.
2. Description of the Related Art
Light source efficiency is a very important parameter in the design of many LC Display devices. This efficiency is the primary determining factor in the time required between recharging periods for a battery of a given size. These devices include lap-top personal computers as well as many metering and indicating devices. The low efficiency of the light source of these devices results in increased size, weight and expense of the units. Thus, much work has been done on increasing the efficiency of the light sources and the piping of the light to the LCD panel. Any significant increase of the efficiency of the display lighting system, while maintaining a flat or relatively thin profile, is of considerable importance.
Two primary approaches have been taken. One approach is that of the high efficiency, serpentine fluorescent lamp imbedded in a plastic light pipe. This structure is placed on the rear of the LCD, with a mirror backing the structure to reflect light toward the LCD panel. The serpentine fluorescent lamps of this structure are expensive, but the improved efficiency, with its associated benefits are often judged to be of sufficient importance to make the expense worthwhile. A second and more popular approach, uses small, linear fluorescent light bulbs, mounted on one or two edges of the display. The light from the bulbs is "light piped" through a thick plastic light pipe across the rear of the LCD panel, where light emanating from the light pipe illuminates the rear of the LCD panel. This structure is also backed with a mirror to improve efficiency and the back sides of the bulbs will usually have reflectors to collect and direct wayward light to the light pipe.
Both of these systems flood the rear of the LCD with white light, which is then polarized, wherein about 60% of the light is lost. The polarized light entering the rear of the panel, is further apertured by a inactive area surrounding each cell (sub-pixel element) of the LCD. For a color display the light arriving at each cell must pass through a color filtering element, which further reduces the light by two thirds. The net result is that a maximum of about 7% of the light passing out of the light pipe to the display, will pass through a cell to a possible viewer. Further, if the LCD is of the active addressing type, then the instantaneous efficiency is further reduced by about one half to less than 4%.
The display technology of lighted signs and most instrumentation displays is well known. Back-lighted signs fall into two classes: The neon sign, which should require no explanation; and the back-lighted translucent sign, which is equally familiar. Instrumentation displays are similarly familiar. The most common instrumentation display is the analog meter movement, which requires no explanation. Similarly simple and common is the back-lighted panel, wherein specific indicators are switched "on" by activating a light source to illuminate a translucent display window of the indicator. These display means are generally inefficient and limited to a single function, which requires a number of such displays to produce the desired communication.
Additionally, light emitting artwork, usually found in the rendering of religious scenes, has generally used the same technology as that of the back-lighted sign or the stain-glass window.
The poor efficiency of the light pipes of the prior art is greatly improved by the methods of the present invention, wherein virtually all the light passing out of the light pipe, can pass through the LCD cells to the viewer. The novel light pipe is also considerably thinner than the existing designs. The new design appears to eliminate a number of process steps in the fabrication of a LCD panel, as well. The methods of this invention also promise a new dimension of lighted artwork in the home and place of business.
The invention utilizes many optical properties of Cholestric Liquid Crystals, more recently known as Chiral Liquid Crystal (CLC) or Chiral Nematic liquid Crystal (CN-LC) materials. The materials were invented by Adams et al, U.S. Pat. No. 3,679,290 and refined by Schadt & Funfschilling and Maurer, c.f. Schadt & Funfschilling, 1990 Jpn. J. Appl. Phys., vol. 29, No. 10, pp 1974-1984, New liquid Crystal Polarized Color Projection Principle, and Maurer, SID 90 Digest, 1990, pp 110-113, Polarizing Color Filters Made From Cholesteric LC-Silicones. These materials have the optical property that depending upon the direction and rate of twist (pitch) of the molecules in the LC structure, will reflect one color and handedness of substantially Circularly Polarized light (CPL), while transmitting all other colors and handednesses of the incident light. In this way the light may subsequently interact with other species of the Liquid Crystal (LC), i.e. those having different reflective color and/or handedness. Thus, these materials, when properly deposited form spectral CLC filter/mirrors, which can be deposited in series, even upon each other, following the teachings of Maurer. In so doing, the filter/mirrors can reflect a broad spectrum of light, ranging from within the Infra Red region, well into the Ultra Violet spectrum following the teachings of Adams et al.
The color of the light reflected is related to the pitch of the LC structure, where the center wavelength color of the light reflected corresponds to that color having a average wavelength equal to the pitch of the LC structure, using the average propagation velocity in the LC to calculate that wavelength. The handedness of the reflected CPL corresponds to the direction of the twist in the LC structure. The spectral distribution of the reflected CPL is related to the birefringence of the LC structure. The reflected light at the center wavelength of the spectral distribution is near perfectly circularly polarized, while other wavelengths in the distribution are slightly elliptically polarized, with the degree of ellipticity increasing toward the edges of the distribution.
Adams states in patent, U.S. Pat. No. 3,679,290, that the CLC films will reflect virtually 100% of the light of one handedness and color of Circularly Polarized Light (CPL), with the absorption of the light by the CLC films usually being negligible. In the present application this property is very important, since each ray of light will experience multiple reflections from the CLC films. Such data that is available on the reflection of CPL from CLC films, suggests that the reflection efficiency of about 99% is equal to or greater than the best specular reflectors. Adams also states in the same patent, that the thickness of the individual film layers are preferably between 0.5 to 20 microns and the ideal film thickness is 3 to 10 microns.
FIG. 1 illustrates the reflective characteristics of three species of CLC filter/mirror materials to unpolarized light. If the light is circularly polarized and of an orientation which is reflected by the filter/mirror materials, then virtually all the CPL is reflected. These CLC filter/mirror materials can be deposited one upon the other to reflect the entire visible spectrum or any portion of the spectrum in one or both polarization orientations. Using only two of the filter/mirrors and polarized light of one handedness, a portion of the spectrum is transmitted through the CLC filter/mirror layers, as is illustrated in FIG. 2. In this illustration much of the light reflected by filter/mirror 2 is transmitted by the two CLC filter/mirror (layers) of FIG. 2.
FIG. 3 illustrates the normalized spectral distributions of the phosphors of a fluorescent light source (bulb) used to backlight a LCD or a sign of the invention. This is typical of bulbs used to backlight most LCD displays. As illustrated the bulb has three phosphors, producing light in three primary colors 4, 5 & 6. The light is judged by the viewer to be white light, although it lacks any continuity in spectral distribution. This is because the light stimulates the three color receptors (cones) of the human eye. The three peaks of the spectral distributions are judged to be blue 4, green 5 and red 6.